1. Field of the Invention
The present invention generally relates to switch units and, more particularly, to a switch unit that illuminates a switch operating position.
2. Description of the Related Art
In order to improve operability in dark places such as nighttime, portable electronic devices typified by mobile phones, for example, are provided with an illumination function of illuminating the vicinities of positions at which key tops are provided. Japanese Laid-Open Patent Application No. 2003-77366, for example, discloses a switch unit provided with such an illumination function. FIG. 1 shows a conventional switch unit 50 provided with such an illumination function.
The switch unit 50 generally includes a first sheet member 52 and a second sheet member 53. The first sheet member 52 includes a cover sheet 54 and dome movable contacts 55. The cover sheet 54 is a sheet made of an insulating resin and has a function of protecting the dome movable contacts 55. The dome movable contacts 55 are made of a conductive metal and are formed into dome-like shapes. When pressed, the dome movable contacts 55 elastically move in up and down directions in FIG. 1.
As shown in FIG. 1, the first sheet member 52 is provided on a wiring substrate 56, thereby forming push button switches. The wiring substrate 56 includes ring fixed contacts 58 and fixed contacts 57 formed in the center positions of the ring fixed contacts 58. The outer peripheries of the dome movable contacts 55 are connected to the ring fixed contacts 58. Thus, when the dome movable contacts 55 are pressed and moved, and the center portions thereof contact the fixed contacts 57, the fixed contacts 57 and the ring fixed contacts 58 are electrically connected via the dome movable contacts 55. Hence, the push button switches are formed by the dome movable contacts 55, the fixed contacts 57, and the ring fixed contacts 58.
The second sheet member 53 includes a plurality of illumination parts 60 formed between a first base member 61 and a second base member 62. Each of the illumination parts 60 is formed by a light-emitting part 63 and first and second transparent electrodes 64 and 65 interposing the light-emitting part 63 therebetween. The light-emitting part 63 emits light by supplying power thereto from each of the first and second transparent electrodes 64 and 65. The first and second transparent electrodes 64 and 65 are formed by an indium-tin oxide (hereinafter referred to as “the ITO”) that is transparent and having conductivity. The reference numeral 70 designates an air pathway forming spacer.
However, since the conventional switch unit 50 uses the first and second transparent electrodes 64 and 65 made of the ITO as the electrodes for supplying power to the light-emitting parts 63, there are problems in that the thickness of the second sheet member 53 is increased, which prevents reduction of the thickness of the switch unit 50, and that the positions pressed by a user when using the switch unit 50 cannot be directly illuminated.
A description is given below of each of the above-mentioned problems.
First, a description is given of the problem that the thickness of the second sheet member 53 is increased. The first and second transparent electrodes 64 and 65 are formed by depositing the ITO on the base members 61 and 62, respectively. It is preferable that the first and second transparent electrodes 64 and 65 have low resistance in terms of ensuring power supply to the light-emitting parts 63 and saving power. In these respects, the ITO has low resistance for a transparent electrode material and is a material having a good electric property.
However, since the base members 61 and 62 are made of resin such as PET (polyethylene terephthalate), in order to achieve the good electric property of the ITO, it is necessary for the base members 61 and 62 to have a thickness with which the base members 61 and 62 do not warp at the time of deposition. Hence, in the conventional switch unit 50, there is a problem in that the thickness and rigidity of the second sheet member 53 are increased, which results in increases in the thickness and rigidity of the switch unit 50.
As mentioned above, it is preferred as is generally known that the size and thickness of a portable electric device typified by a mobile phone be reduced. Thus, there is a problem in that the increase in the thickness of the switch unit 50 prevents reduction of the size and thickness of a portable electronic device that incorporates the switch unit 50 therein. In addition, if the thickness of the switch unit 50 is increased, then the rigidity of the switch unit 50 is increased. When the rigidity of the switch unit 50 is increased, it becomes difficult or impossible to make the switch unit 50 to be flexibly deformed as a flexible substrate, which causes a problem of poor mounting to the portable electronic device.
On the other hand, since the ITO is a metal oxide (indium-tin oxide) and a thin film, when the ITO is continuously subjected to deformation and biasing, metal fatigue may occur. Hence, in the case where each of the first and second transparent electrodes 64 and 65 formed by the ITO is provided directly on one of the dome movable contacts 55 and is deformed and biased every time the switch is operated, it is conceivable that the first and second transparent electrodes 64 and 65 may be damaged in a relatively short period of time. Hence, conventionally, as shown in FIG. 1, the transparent electrodes 64 and 65 are not provided directly on the dome movable contacts 55. That is, the first and second transparent electrodes 64 and 65 are provided on the outer peripheries of the dome movable contacts 55.
On the other hand, when operating the switch unit 50 in a dark place, it is preferable in terms of operability to illuminate the position at which the operation is directly performed, that is, the position at which the dome movable contact 55 is provided. However, because of the above-mentioned reasons, it has been conventionally difficult or impossible to directly illuminate the position that is pressed when operating the switch unit 50. Hence, usability of the switch unit 50 has not been fully satisfactory.